We are studying the regulation of chromosome segregation and nuclear division in budding yeast. Nuclear division is a complex process that involves the duplication of chromosomes, their accurate segregation and the remodeling of the yeast nucleus, which remains intact throughout mitosis. In the past year our work has focused on three main aspects of this process: the identification and characterization of proteins involved in nuclear positioning, the isolation of novel mitotic regulators, and the study of proteins and structures that affect nuclear shape. Below is a summary of each of these research projects:[unreadable] [unreadable] 1. Nuclear positioning in budding yeast. We previously discovered a regulatory pathway that is responsible for generating a force that pulls the nucleus towards the cortex of the mother cell (Ross and Cohen-Fix, Dev Cell 2004). In an attempt to identify the underlying mechanism of this process, we screened through a collection of mutants to determine if any were defective in this mother-bound force. One of the mutations isolated was in the ASE1 gene, which codes for a spindle associated protein. The previously known function of Ase1p, a homolog of the mammalian PRC1, is to stabilize the spindle midzone. Thus, the observation that Ase1p could affect nuclear positioning, a process that is thought to occur outside the nucleus, was surprising. We found that Ase1p is involved in regulating the number and orientation of astral microtubules, which are the vehicle through which force is exerted on the nucleus. This is a novel function for Ase1p that is likely be relevant to the mechanism by which nuclei are positioned in higher eukaryotes.[unreadable] [unreadable] 2. Apq12p, a novel protein involved in cell cycle regulation. In a previous study (Sarin et al, Genetics 2004) we searched for novel functions that are needed for chromosome segregation. Through this screen we isolated APQ12, a gene that codes for an ER-associated protein of unknown function. The involvement of the ER protein in cell cycle regulation is intriguing, as this is one of the first examples that links ER function with cell cycle progression. Interestingly, the absence of Apq12p leads to intracellular damage that activated the spindle assembly checkpoint pathway, suggesting that Apq12p is involved in spindle or kinetochore function. We are in the process of determining the molecular function of Apq12 and identifying interacting proteins that acts in the Apq12 pathway. [unreadable] [unreadable] 3. The mechanisms and structures that control nuclear shape. Abnormalities in nuclear shape are one of the hallmarks of cancerous cells, but the consequences of shape change, and its relevance to the etiology of the disease, are unknown. To gain insights into how nuclear shape maintained, we used a yeast mutant strain that exhibits an abnormally shaped nucleus. This strain is defective the in regulation of phospholipids biosynthesis, through a mutation in the SPO7 gene, thereby causing nuclear membrane proliferation. Importantly, however, we found that the expansion of the nuclear membrane is not uniform but is confined to the region that is associated with the yeast nucleolus. Our investigations (Campbell et al, Mol Biol Cell 2006) have led us to propose that the yeast nuclear membrane has distinct domains that differ in their susceptibility to membrane expansion. Furthermore, our findings suggest that there is a structure that restricts nuclear membrane expansion in the nuclear domain that contains the bulk of the chromosomal DNA. This structure, which is likely to be essential for maintaining nuclear shape, is the subject of ongoing investigations.